[1.0.0-alpha2] Don't show icon source if no icons

Problem/Motivation

IconSource is available even if Drupal Core's Icon API detect no icons. This is confusing because the user hopes to get icons but the

Proposed resolution

Do we need to add an isApplicable() method to UI Patterns's SourceInterface?

Like it is already the case in Drupal Core's:

• SectionStorageInterface
• WidgetInterface
• FormatterInterface